Power Frequency Converter Based on Integral-cycle Triggering Mode of Thyristors with Multi-control Periods

Conventional integral-cycle control was abandoned as an AC motor speed controller many years ago because the unbalanced sets of sub-harmonic and higher-order harmonic voltages generated by this type of control create severe problems for AC machines, causing excessive heat, mechanical vibrations, and noise. On the other hand, integral-cycle control has many advantages over phase-angle control as it generates fewer harmonics, reduces the electromagnetic interference, and possesses all the advantages of zero-voltage switching techniques. In the present work, an attempt is made to develop an integral-cycle control based power frequency changer using multi-conduction and multi-control periods that can compete with the conventional power frequency changers used to drive AC machines, such as inverters and cycloconverters. A comprehensive mathematical analysis for the generated voltage waveforms is carried out as well as a prototype experimental model of the frequency changer has been designed and built in the laboratory to investigate its performance with several types of load. The problem of phase unbalance associated with integral-cycle control when it is used with three-phase system is also studied, and an attempt is made to solve this problem by using different phase-angle triggering and phase-shifting techniques.